Hydrogen production by partial oxidation of methanol over bimetallic Au–Ru/Fe2O3 catalysts

Hydrogen production by partial oxidation of methanol (POM) was investigated over Au–Ru/Fe₂O₃ catalyst, prepared by deposition–precipitation. The activity of Au–Ru/Fe₂O₃ catalyst was compared with bulk Fe₂O₃, Au/Fe₂O₃ and Ru/Fe₂O₃ catalysts. The reaction parameters, such as O₂/CH₃OH molar ratio, calcination temperature and reaction temperature were optimized. The catalysts were characterized by ICP, XRD, TEM and TPR analyses. The catalytic activity towards hydrogen formation is found to be higher over the bimetallic Au–Ru/Fe₂O₃ catalyst compared to the monometallic Au/Fe₂O₃ and Ru/Fe₂O₃ catalysts. Bulk Fe₂O₃ showed negligible activity towards hydrogen formation. The enhanced activity and stability of the bimetallic Au–Ru/Fe₂O₃ catalyst has been explained in terms of strong metal–metal and metal–support interactions. The catalytic activity was found to depend on the partial pressure of oxygen, which also plays an important role in determining the product distribution. The catalytic behavior at various calcination temperatures suggests that chemical state of the support and particle size of Au and Ru plays an important role. The optimum calcination temperature for hydrogen selectivity is 673 K. The catalytic performance at various reaction temperatures, between 433 and 553 K shows that complete consumption of oxygen is observed at 493 K. Methanol conversion increases with rise in temperature and attains 100% at 523 K; hydrogen selectivity also increases with rise in temperature and reaches 92% at 553 K. The overall reactions involved are suggested as consecutive methanol combustion, partial oxidation, steam reforming and decomposition. CO produced by methanol decomposition is subsequently transformed into CO₂ by the water gas shift and CO oxidation reactions.     

Mechanism of deactivation of Au/Fe2O3 catalysts under water–gas shift conditions

The stability of Au supported catalysts for the water–gas shift reaction was studied. Two types of continuous flow experiments were performed, i.e. temperature-programmed and long-term isothermal stability test. The highest initial rate was obtained for catalysts used without any calcination or other high-temperature treatment. The continuous flow experiments showed that all Au/Fe₂O₃ catalysts deactivated under water–gas shift conditions. The deactivation trend occurred independently on the Au loading, the method of preparation, calcination or high-temperature treatment. The various causes on the deactivation, i.e. the formation of carbon-containing species, the change of Au particles or changes of the support were investigated in terms of DRIFTS coupled with MS, TGA, TEM, N₂ physisorption, ICP, and XRD. Even though stable carbonate and carbonyl surface species were found on the spent catalysts, the quantity of these species indicated that they are not the main cause of the deactivation. Furthermore, the agglomeration of the Au particles was not severe and was not observed for all Au catalysts. The deactivation of Au/Fe₂O₃ was mainly caused by the change of the support. A significant reduction of the surface area of the support is taking place during the water–gas shift reaction as a function of time on stream. This decrease of the surface area can almost solely explain the decrease on the catalytic activity.     

The Preparation of Highly Dispersed Au/Al2O3 by Aqueous Impregnation

In this work, we report an impregnation method for preparing Au supported on alumina from HAuCl₄. In the literature, impregnation under acidic conditions has been found to lead to poor dispersions of Au and the resulting catalysts are not as active as those prepared by deposition-precipitation. To overcome these problems, we have developed a two-step procedure: in the first step, the acidified Au solution is contacted with alumina to adsorb the Au chloride on the alumina. After washing off the excess Au precursor, we treat the solid with a strong base to convert the chloride to an absorbed hydroxide. Drying and calcination at 400 °C yields a catalyst with Au particles having a number average diameter of 2.4 nm. The reactivity for CO oxidation at room temperature is comparable to catalysts prepared by deposition-precipitation. These catalysts are stable to hydrothermal sintering, with average particle size around 4 nm after sintering in 10 mol% H₂O at 600 °C for 100 h. This work shows that stable Au/Al₂O₃ catalysts having a high reactivity for CO oxidation can be prepared by impregnation under acidic conditions.     

The effect of high-temperature pre-treatment and water on the low temperature CO oxidation with Au/Fe2O3 catalysts

The low temperature activity of Au/Fe₂O₃ catalysts towards CO oxidation was examined with respect to the temperature of pre-treatment and presence of water. The activity of all the prepared catalysts decreased as a result of a high temperature treatment (HTT) at 400 °C. The inclusion of water in the gas stream significantly enhanced the oxidation of CO at room temperature. When tested under water gas shift reaction (WGSR) conditions, significantly higher temperatures were required to convert CO to CO₂, thereby excluding the possibility of the WGSR during CO oxidation in the presence of H₂O at room temperature. The loss of activity for CO oxidation is attributed to the loss of hydroxyl groups and reduction of Au³⁺ to metallic gold during HTT. The observations are consistent with the model for hydroxyl promotion of the decomposition of a carbonate intermediate by transformation to less stable bicarbonate.     

CO preferential oxidation over Au/MnOx-CeO2 catalysts prepared with ultrasonic assistance: Effect of calcination temperature

The Au/MnO_x-CeO₂ catalysts used for CO preferential oxidation were prepared by deposition-precipitation with ultrasonic assistance. The effect of calcination temperature (150-350 °C) on the structures and catalytic performance of the catalysts was systematically investigated. It is found that the catalyst Au/MnO_x-CeO₂ calcined at 250 °C exhibits the best catalytic performance, giving not only the highest CO conversion of 90.9% but also the highest selectivity of oxygen to CO₂ at 120 °C. The results of XRD, TEM and XPS indicate that this catalyst possesses the smallest particle size, the highest dispersion of Au species and the largest amount of surface adsorbed oxygen species, which are favorable to CO oxidation. The H₂-TPR results reveal that the selectivity of oxygen to CO₂ is mainly determined by the reducibility of Au species in the catalysts. The strong interaction between Au species and the support in Au/MnO_x-CeO₂-250 decreases its capability for H₂ dissociation and oxidation, leading to high selectivity of oxygen to CO₂.     

Methanobactin-Mediated Synthesis of Gold Nanoparticles Supported over Al2O3 toward an Efficient Catalyst for Glucose Oxidation

Methanobactin (Mb) is a copper-binding peptide that appears to function as an agent for copper sequestration and uptake in methanotrophs. Mb can also bind and reduce Au(III) to Au(0). In this paper, Au/Al₂O₃ catalysts prepared by a novel incipient wetness-Mb-mediated bioreduction method were used for glucose oxidation. The catalysts were characterized, and the analysis revealed that very small gold nanoparticles with a particle size <4 nm were prepared by the incipient wetness-Mb-mediated bioreduction method, even at 1.0% Au loading (w/w). The influence of Au loading, calcination temperature and calcination time on the specific activity of Au/Al₂O₃ catalysts was systematically investigated. Experimental results showed that decomposing the Mb molecules properly by calcinations can enhance the specific activity of Au/Al₂O₃ catalysts, though they acted as reductant and protective agents during the catalyst preparation. Au/Al₂O₃ catalysts synthesized by the method exhibited optimum specific activity under operational synthesis conditions of Au loading of 1.0 wt % and calcined at 450 °C for 2 h. The catalysts were reused eight times, without a significant decrease in specific activity. To our knowledge, this is the first attempt at the preparation of Au/Al₂O₃ catalysts by Mb-mediated in situ synthesis of gold nanoparticles.     

The preparation of Au/CeO2 catalysts and their activities for low-temperature CO oxidation

Au/CeO₂ catalysts prepared by co-precipitation (CP) and deposition-precipitation (DP) methods were tested for low temperature CO oxidation reaction. The structural characters and redox features of the catalysts were investigated by XRD, XPS and H₂-TPR. Their catalytic performances for low temperature CO oxidation were studied by means of a microreactor -GC system. It showed that the catalytic activities of Au/CeO₂ catalysts greatly depended on the preparation method. The catalysts prepared by DP method exhibited a surprisingly higher activity towards CO oxidation than that prepared by CP method. This may arise from the differences in the particle sizes of Au and redox properties of the catalysts. The low Au loading and the resistance to high temperature of DP-prepared catalyst made it more applicable.     

Highly reproducible syntheses of active Au/TiO2 catalysts for CO oxidation by deposition–precipitation or impregnation

Gold catalysts supported on TiO₂ were prepared by a deposition–precipitation (DP) method to investigate how highly reproducible performance of the gold catalysts in CO oxidation can be achieved. A protocol was established for synthesizing identically performing catalysts by different operators. The results show that for this synthesis route, the calcination step is not needed to form highly active Au/TiO₂ catalysts, but leads to decreased activity. Improved catalytic activity was observed when a high solution pH was adjusted during the precipitation. Surprisingly, wet impregnation followed by ammonia steam treatment and a washing step with water also leads to Au/TiO₂ with 2- to 4-nm individual gold particles highly dispersed on the TiO₂ surface. In addition, this catalyst is active for room temperature CO oxidation. The temperature for 50% conversion of CO is below 25 °C, which is comparable to that of the gold catalyst prepared by the DP method. Therefore, contrary to reports in the literature, the impregnation method can be used in the preparation of high-activity gold catalysts.     

An investigation of the activity of coprecipitated gold catalysts for methane oxidation

Coprecipitated Au on transition metal oxide catalysts have been tested for their activity toward methane oxidation. Catalyst activities fall in the order Au/Co₃O₄>Au/NiO> Au/MnO_x> Au/Fe₂O₃ > Au/CeO. The Au/Co₃O₄ catalyst is active just below about 250°C. The catalysts are proposed to have more than one type of reactive site since the supports are also active at higher temperatures. Analysis of spent catalysts with X-ray photoelectron spectroscopy indicates that Au exists in at least two oxidation states on some of them, a reduced state and an oxidized state. The activity for methane oxidation increases with increasing oxidation of Auin the oxidized state.     

Complete oxidation of 2-propanol over gold-based catalysts supported on metal oxides

This paper concerns the preparation of metal oxide-supported gold catalysts and their application to 2-propanol abatement in order to lower the light off temperature. Catalytic oxidation of 2-propanol was investigated on Au/CeO₂, Au/Fe₂O₃, Au/TiO₂ and Au/Al₂O₃ catalysts prepared from the deposition–precipitation (DP) method. The catalysts are characterized by XRD (X-ray diffraction), BET (Brunner–Emmett–Teller), TEM (transmission electron microscopy), NH₃-TPD (NH₃-temperature programmed desorption), H₂-TPR (H₂-temperature programmed reduction), ICP-AES (inductively coupled plasma-atomic emission spectroscopy) and XPS (X-ray photoelectron spectroscopy) techniques. The catalytic activity of Au/metal oxide samples towards the deep oxidation of 2-propanol to CO₂ and water has been found to be strongly dependent on the kind of supports, the amount of gold loading, the calcination temperature and the moisture content in the feed.     

Influence of modifying additives on the catalytic activity and stability of Au/Fe2O3–MO x catalysts for the WGS reaction

The activity and stability of Au/Fe₂O₃–MO_x catalysts (M = Zr, Mg, Ca, Ni, La, Cu, Zn, Al, Ba, Cr, Co, Ce, Mo, Bi, Ti, Mn) in water-gas shift reaction were investigated extensively. The WGS activity and stability of Au/Fe₂O₃ is improved significantly upon addition of ZrO₂ and to a lesser extend MgO, CaO, NiO, La₂O₃, Cr₂O₃, CuO. In contrast, Bi, Ti and Mn oxides seriously decrease the catalytic activity while additions of Zn, Al, Ba, Co, Ce and Mo oxides do not influence evidently the catalytic activity and its stability. Based on the characterization using the methods of BET-surface area and pore structure XRF, XRD, and H₂–TPR for some of as-prepared and spent samples, it could be concluded that the catalytic activity of gold catalysts supported on composite oxide of Fe₂O₃–MO_x depends not only on the dispersion of the gold particles but also on the reduction property of composite oxide supports, regardless of the fluctuation of gold loading and some change of specific surface area and pore structure due to introduction of the modifying metal oxides. The improvement of catalytic stability may be attributed to the comparative stabilization of high dispersion of gold particles and uneasily sintering of Fe₃O₄ crystallites during the catalytic operation. However, the chemical (electronic) effects exerted by the modifying addition of metal oxides on the catalytic performance of gold catalyst may not be ruled out.     

Gold-base catalysts supported on carbonate for low-temperature CO oxidation

Au/MeCO₃ (Me = Ca, Sr, Ba) catalysts prepared by co-precipitation method were studied for low-temperature CO oxidation in the presence/absence of water in the feed stream. The type of support and the calcination temperatures have considerable effect on the activity of the catalysts. Among them, Au/BaCO₃ calcined at 473 K shows the highest activity, full conversion of CO can be obtained at ambient reaction temperature in the presence of moisture. The addition of water vapor in feed stream has positive influence on the activity of Au/MeCO₃ catalysts for CO oxidation, which might be due to water can participate CO oxidation reaction directly.     

Effect of Aging Time and Calcination on the Preferential Oxidation of CO Over Au Supported on Doped Ceria

Au/CeLaO_x mixed oxide catalysts containing 0.6–1.0 wt% Au were prepared by co-precipitation with Na₂CO₃. BET surface areas ranged from 15 to 45 m²/g depending on aging time (precipitation time) and calcination conditions. The differences in the activity of the catalysts for preferential oxidation (PROX) of CO are ascribed to the differences in the metal loading, Ce/La ratio and support crystallinity, chloride content, and the resultant effect on the reduction properties of the catalysts. The catalysts did not require activation in H₂ prior to reaction. The temperature at which the catalysts exhibit significant activity correlates with the temperature of reduction, indicating that reduction of the metal and support is important for high activity.     

Catalytic Combustion of Benzene over Au Supported on Ceria and Vanadia Promoted Ceria

Complete oxidation of benzene over Au/CeO₂ and Au/V₂O₅/CeO₂ catalysts were studied. Gold was supported on CeO₂ from different sources by deposition precipitation method. The catalysts were characterized by XRD, BET, X-ray photoelectron spectroscopy, TEM, and H₂-TPR techniques. The catalytic activity toward the complete oxidation of benzene to CO₂ and water were strongly dependent on the kind of CeO₂ sources, loading amount of gold, modified amount of vanadia, and calcination temperature. High activities were obtained on 1% Au/CeO₂ and 2% vanadia modified Au/CeO₂ catalysts calcined at 300 °C. The nanometer size of Au particle and interaction between Au, V₂O₅, and CeO₂ play important roles in determining the activity of benzene complete oxidation.     

CO2 hydrogenation to methanol over Cu/ZnO/ZrO2 catalysts prepared via a route of solid-state reaction

Cu/ZnO/ZrO₂ catalysts were prepared by a route of solid-state reaction and tested for the synthesis of methanol from CO₂ hydrogenation. The effects of calcination temperature on the physicochemical properties of as-prepared catalysts were investigated by N₂ adsorption, XRD, TEM, N₂O titration and H₂-TPR techniques. The results show that the dispersion of copper species decreases with the increase in calcination temperature. Meanwhile, the phase transformation of zirconia from tetragonal to monoclinic was observed. The highest activity was achieved over the catalyst calcined at 400 °C. This method is a promising alternative for the preparation of highly efficient Cu/ZnO/ZrO₂ catalysts.     

Hydroformylation of olefins by Au/Co3O4 catalysts

The hydroformylation of olefins over supported gold catalysts in an autoclave reactor under mild conditions (100–140 °C, 3–5 MPa) has been studied. Over Au/AC (activated carbon), Au/PVP (polyvinylpyrrolidone), Au/Al₂O₃, Au/TiO₂, Au/Fe₂O₃, Au/ZnO, Au/CeO₂ and Co₃O₄, 1-olefin mainly remained unchanged and the major products were isomerized olefins or hydrogenated paraffin. In contrast, Au nanoparticles deposited on Co₃O₄ led to remarkably high catalytic activities in hydroformylation reaction with selectivities above 85% to desired aldehydes. The hydroformylation of olefins proceeds preferentially at temperatures below 140 °C, above which the reactions of olefins gradually shifted to isomerization and then to hydrogenation. It appeared that the activity and selectivity of hydroformylation reaction strongly depend on the molecular structure of olefins, which could be ascribed to steric constraints as internal olefins are relatively inappropriate to form alkyl group and subsequent acyl group by insertion of CO. The Au/Co₃O₄ catalyst can be recycled by simple decantation with slight decrease in catalytic activity along with an increase in recycle times, which is a great advantage over homogeneous catalysts. The role of gold nanoparticles can be assumed to dissociate hydrogen molecule into atomic species which reduce Co₃O₄ to Co metal under mild reaction conditions.     

Preferential Oxidation of CO in H2-Rich Gas at Low Temperatures over Au Nanoparticles Supported on Metal Oxides

We investigated Au catalysts supported on TiO₂, Fe₂O₃, and ZnO for their preferential oxidation of CO in a H₂-rich atmosphere. Both full conversion and selectivity were achieved over Au/Fe₂O₃ and Au/ZnO around room temperature, but at higher temperatures the CO conversion was suppressed due to competition between CO and H₂.     

Au/Ce1−xZrxO2 as effective catalysts for low-temperature CO oxidation

The physico-chemical properties and activity of Ce-Zr mixed oxides, CeO₂ and ZrO₂ in CO oxidation have been studied considering both their usefulness as supports for Au nanoparticles and their contribution to the reaction. A series of Ce_1−xZr_xO₂ (x = 0, 0.25, 0.5, 0.75, 1) oxides has been prepared by sol–gel like method and tested in CO oxidation. Highly uniform, nanosized, Ce-Zr solid solutions were obtained. The activity of mixed oxides in CO oxidation was found to be dependent on Ce/Zr molar ratio and related to their reducibility and/or oxygen mobility. CeO₂ and Ce_0.75Zr_0.25O₂, characterized by the cubic crystalline phase show the highest activity in CO oxidation. It suggests that the presence of a cubic crystalline phase in Ce-Zr solid solution improves its catalytic activity in CO oxidation. The relation between the physico-chemical properties of the supports and the catalytic performance of Au/Ce_1−xZr_xO₂ catalysts in CO oxidation reaction has been investigated. Gold was deposited by the direct anionic exchange (DAE) method. The role of the support in the creation of catalytic performance of supported Au nanoparticles in CO oxidation was significant. A direct correlation between activity and catalysts reducibility was observed. Ceria, which is susceptible to the reduction at the lowest temperature, in the presence of highly dispersed Au nanoparticles, appears to be responsible for the activity of the studied catalysts. CeO₂-ZrO₂ mixed oxides are promising supports for Au nanoparticles in CO oxidation whose activity is found to be dependent on Ce/Zr molar ratio.     

Study of Gold Species in Iron-Oxide-Supported Gold Catalysts Derived from Gold-Phosphine Complex Au(PPh3)(NO3) and As-Precipitated Wet Fe(OH)3*

Iron-oxide-supported gold catalysts were prepared by supporting a Au phosphine complex Au(PPh₃)(NO₃) on as-precipitated wet iron hydroxide Fe(OH)₃^*, followed by temperature-programmed calcination. The Au/Fe(OH)₃^*catalysts calcined at the temperatures 573–773 K showed extremely high catalytic performance for CO oxidation at temperatures as low as 203–253 K. Interaction of the Au(PPh₃)(NO₃) gold precursor with the Fe(OH)₃^*upon supporting, transformation of the precursor during the heat treatments, and state of the gold in the catalysts were studied by FT-IR, XRD, TEM, XPS, and EXAFS. The gold precursor dissociated on the Fe(OH)₃^*surface to produce [Au(PPh₃)]⁺species which partially decomposed at 473 K and was transformed to small gold metallic particles with coordination numbers of 7.4–8.0 for Au-Au bond at calcination temperatures ≥573 K. In contrast, decomposition of the gold complex over crystalline Fe₂O₃^*resulted in large gold particles. The Au/Fe₂O₃^*sample was inactive at 203–253 K and exhibited very low activity for CO oxidation at room temperature. The efficiency of the as-precipitated wet Fe(OH)₃^*as a support is explained in terms of a higher stability of [Au(PPh₃)]⁺on the Fe(OH)₃^*as compared to the Fe₂O₃^*due to more effective interaction of the Au species with OH groups and defects of the amorphous Fe(OH)₃^*surface. The results demonstrate the importance of support–metal precursor interactions, both upon supporting and during calcination, in the formation of highly active catalysts with small Au particles for low-temperature CO oxidation.     

Promotional effect of hydroxyl on the aqueous phase oxidation of carbon monoxide and glycerol over supported Au catalysts

Gold particles supported on carbon and titania were explored as catalysts for oxidation of CO or glycerol by O₂ at room temperature in liquid-phase water. Although Au/carbon catalysts were not active for vapor phase CO oxidation at room temperature, a turnover frequency of 5 s⁻¹ could be achieved with comparable CO concentration in aqueous solution containing 1 M NaOH. The turnover frequency on Au/carbon was a strong function of pH, decreasing by about a factor of 50 when the pH decreased from 14 to 0.3. Evidently, a catalytic oxidation route that was not available in the vapor phase is enabled by operation in the liquid water at high pH. Since Au/titania is active for vapor phase CO oxidation, the role of water, and therefore hydroxyl concentration, is not as significant as that for Au/carbon. Hydrogen peroxide is also produced during CO oxidation over Au in liquid water and increasing the hydroxyl concentration enhances its formation rate. For glycerol oxidation to glyceric acid (C₃) and glycolic acid (C₂) with O₂ (1–10 atm) at 308–333 K over supported Au particles, high pH is required for catalysis to occur. Similar to CO oxidation in liquid water, H₂O₂ is also produced during glycerol oxidation at high pH. The formation of the C-C cleavage product glycolic acid is attributed to peroxide in the reaction.